1. Field of the Invention
The present invention relates to a soft-start circuit used in a power supply circuit to prevent an excessive increase in output current to a load during start-up, and a power supply circuit including the soft-start circuit.
2. Discussion of the Background
Generally, power supply circuits configured as integrated circuits (IC) that adjust the gradient of a soft-start voltage by changing the capacity of a capacitor provided outside the integrated circuits are widely used. Because a capacitor provided inside the integrated circuit requires a large area, it is unrealistic to increase the capacity of the capacitor so as to reduce the gradient of the soft-start voltage. Consequently, the capacitors have to be positioned outside the integrated circuit.
For example, one related-art power supply circuit includes a soft-start circuit that generates a single slope voltage and outputs a voltage that follows the slope voltage as a soft-start voltage.
In this circuit, when the soft-start circuit generates the slope voltage by charging the capacitor (external capacitor) positioned outside the integrated circuit from a power source and outputs the soft-start voltage that follows the slope voltage, the gradient of the soft-start voltage can be adjusted by changing the capacity of the external capacitor.
However, this configuration always requires the external capacitor when the gradient of the soft-start voltage is adjusted to any given gradient. Therefore, it can happen that, when the capacitor is insulated due to a defect in manufacture of the circuit or the like, the gradient of the soft-start voltage becomes steeper, and a large current flows to the load connected to a power supply circuit configured as an IC, thus causing failure of the load.
In another related-art power supply circuit including a soft-start circuit, when the soft-start circuit generates the slope voltage by charging a capacitor provided inside the integrated circuit (hereinafter “internal capacitor”) from a power source and outputs the soft-start voltage that follows the slope voltage, the gradient of the soft-start voltage is reduced by decreasing the current from the current source that charges the capacitor without increasing the capacity of the internal capacitor.
In this configuration, the external capacitor is not required, and thus the number of external components can be reduced. However, because no capacitor is provided outside the integrated circuit, the gradient of the slope voltage cannot be adjusted.
In view of the foregoing, there is market demand for power supplies including a soft-start circuit that outputs a soft-start voltage with a milder gradient.